1. Field of the Invention
The present invention relates to a data transfer apparatus and an image forming system.
2. Description of the Related Art
Recently, a high speed serial transmission line, whose transmission speed exceeds several Gbps, capable of transmitting data much faster than Ethernet (registered trademark) has been actualized. For this kind of transmission line, a standard of a local In/Out (I/O) adopting a communication method similar to a conventional computer network has been spreading. Thus, a data network for communications within a chip, between chips or between boards of a Large Scale Integration (LSI) by packet communication using a serial transmission line and a switch, similar to the computer network, is becoming to be used in an electric device to be embedded in a system.
Specifically, for a system such as a Personal Computer (PC) system, an embedded system such as an image forming apparatus such as a Multi Function Peripheral (hereinafter, simply referred to as a “MFP”) or the like, a high speed serial communication standard so-called Peripheral Component Interconnect Express (PCI Express (registered trademark) hereinafter, simply referred to as a “PCIe” as well) has been used for transmitting mass data within an apparatus.
The merit for using the PCIe, which is the high speed serial communication standard, is that it is only necessary to increase the number of lanes in order to spread band so that it is flexible in increasing and decreasing the band compared with the conventional PCI, which is a parallel communication standard. Further, by using the PCIe, the number of the terminals for the LSI and the number of interconnects on a board can be reduced. Further, as the components for the PCIe are mass produced for PCs, the cost for the components is relatively low.
(Virtual Channel)
For a data transfer system in which plural data transfer apparatuses are connected with each other via a common channel for transferring data, if traffic streams of different kinds are transferred through the same channel, there may be a conflict. When such a conflict occurs, the communication qualities (data transfer rate, continuity or the like) for the traffic streams may not be maintained. Thus, an algorithm of arbitration for a serial interface of the PCIe includes a virtual channel function in which a serial bus is time shared by plural virtual channels so that packet data of the plural traffic streams are separately transferred, and an arbitration function in which priority in issuing packet data is arbitrated for each of the virtual channels. The virtual channel is simply referred to as a “VC” as well, hereinafter. With this, Quality of Service (QoS) is actualized. In the data transfer apparatus that includes such an algorithm, packet data is transferred while the order of transferring the data is adjusted for each of the transactions when the packet data of the plural traffic streams corresponding to the plural virtual channels whose priorities are different are to be transferred by the serial bus at the same time (see Patent Document 1).
For the data transfer apparatus which is to be embedded in a system such as an image forming apparatus such as a MFP, the capacities of buffers of the plural virtual channels are respectively optimized corresponding to the kinds of transactions in accordance with the purpose of the system.
Therefore, as will be explained later in detail, there is a problem when it is necessary for the data transfer apparatus to communicate with another data transfer apparatus which is not configured to form plural virtual channels corresponding to those of the data transfer apparatus, that throughput in transferring data of some kinds of transactions via a virtual channel is lowered.